1. Field of the Invention
The present invention relates generally to systems and methods for noise attenuation, and more particularly to a method and apparatus for attenuating noise through phased accumulation and confinement of compressible flow mass and noise, whereby noise is attenuated by ringdown. The noise attenuation method of the present invention has utility both in exhaust systems and intake systems, and has particular utility in exhaust systems of engines operable under elevated back pressure conditions.
2. Related Art
Sound, including sound noise, is generated by pressure fluctuation in a medium, where the pressure fluctuation propagates through the medium in the form of a pressure wave; the pressure wave transmits acoustic energy. The medium may be solid or fluid, such as liquid, gas or a mixture thereof.
Conventional noise attenuation systems and methods utilize basic sound propagation and dissipation principles to attenuate noise generated by a source, such as the exhaust noise of an engine. Generally, such noise attenuation systems and methods may be characterized as active type or passive type.
Active type noise attenuation systems and methods include noise cancellation pressure waves generated using various electromechanical feed-forward or feed-back control elements and techniques. For example, a source of cancellation sound may be provided in communication with a source of undesirable noise and controlled so as to generate sound/pressure wave fluctuations that are complimentary to the sound/pressure wave fluctuations of the undesirable noise, where the complimentary sound and undesirable noise pressure wave fluctuations are superimposed on each other such that the respective pressure wave fluctuations cancel each other out.
Passive type noise attenuation systems and methods are those whose noise attenuation performance is a function of the geometry and sound absorbing properties of the system components. Sound, that is, acoustic energy transmitted in the form of pressure waves, decays naturally by conversion into heat. This conversion may occur by either one or both of i) molecular relaxation in the bulk of the acoustic propagation medium, and ii) interaction between the pressure wave/medium and any sound absorbing boundaries of the system, such as sound absorbing walls, linings, and the like.
Conventional active type and passive type systems may include one or any number of noise attenuating components or elements, such as pipes, chambers, ducts, reflection walls, projections, perforated structures, and the like, or portions thereof, lined or unlined, variously arranged to provide area discontinuities, impedance discontinuities, reflective surfaces, absorptive surfaces, and the like, for directing, reflecting, absorbing and attenuating noise (acoustic energy/pressure waves).
A discussion of various conventional noise attenuation structures, their operating principles, and various analytical methods, including the transfer matrix approach and the finite-element, boundary element, and acoustical-wave finite-element methods, may be found in Beranek and Ver, “Noise and Vibration Control Engineering; Principles and Applications”, John Wiley & Sons, Inc. (1992).
FIG. 1 schematically illustrates a generic silencer (muffler) 110 utilizing conventional passive type noise attenuating elements and methods. As shown therein, exhaust (a compressible flow mass) and noise from a noise source (shown in phantom) 112, such as an engine, flow through a transmittance path including an inlet 114, a plurality of passive type noise attenuating elements (e.g., tubes, chambers, perforated structures, and the like), and an outlet 116 to an external environment (shown in phantom) 118. As shown by arrows therein, noise (acoustic energy/pressure waves) from the noise source generally is directed and redirected at impedance discontinuities, walls and other structural features, so as to be attenuated.
By design, conventional noise attenuation systems such as the silencer of FIG. 1 feature a continuously open transmittance path for flow of compressible exhaust mass and noise, between the noise source and the external environment. Noise attenuation is achieved through (1) acoustic wave reflection at cross-sectional discontinuities, which impede sound propagation but permit a continuous gross flow of compressible exhaust mass, and (2) acoustic energy dissipation resulting from sound wave interaction with absorptive boundaries or walls. As schematically illustrated in FIG. 1, for example, an acoustic wave (noise) incident at inlet 114 of silencer 110 (see large arrow 115A) is attenuated as it flows through and exits silencer 110 (see small arrow 115B). Attenuation of the acoustic wave is achieved by reflections at impededence discontinuities (see, e.g., arrow 115C) and by absorption, e.g., at absorptive boundary 119 (see successively diminishing arrows 115D, 115E, 115F). Conventional noise attenuation systems have a relatively steady (substantially continuous) gross flow of compressible exhaust mass through a defined transmittance path, where the gross flow experiences superimposed fluctuations at the source (source volume flow cycle) under fixed operating conditions, such as an engine exhaust cycle.
Although conventional noise attenuation systems and methods have utility in many applications, such systems and methods have a drawback in that achievable noise attenuation is limited because the interaction of the propagating sound waves with noise attenuating structures in such conventional systems generally is limited to the time the sound waves (noise) take to propagate through the length of the transmittance path of the noise attenuating system. A need exists for an improved system and method for attenuating noise.